A liquid crystal display which can substitute for a cathode ray tube (CRT) has advantages of relatively light weight and low consumption of electricity. In particular, as a thin film transistor-liquid crystal display (TFT-LCD) operates each pixel independently, response time is relatively short and a high quality of resolution of moving images can be obtained. Accordingly, the TFT LCD has many applications, including, for example, notebook computers, wall hangings, televisions and the like.
For orientation of a liquid crystal, an orientation film is formed from a polymeric material. The polymeric material is coated onto an ITO electrode to form an orientation film, and then the orientation film is rubbed with a rubbing cloth of nylon or rayon, in a so-called "rubbing process". Conventionally, a heat resistant polymeric material such as a polyimide is applied by a spin coating or a printing method, which can be readily carried out by a person of ordinary skill in the art, and the rubbing process follows. This conventional method of preparing the orientation film is generally applied in mass production of LCDs because the polymeric material can be easily and rapidly coated onto the ITO electrode. However, the conventional method has shortcomings, including potential damage to the thin film transistor due to static electricity generated during the rubbing process, and potential adverse effects on the orientation film due to dust or fibers from the rubbing cloth. Inorganic materials such as silica (SiO.sub.2) have been used to form orientation films, but deposition rates can be unacceptably low for mass production processes.
LCDs can be in one of a number of different "modes", including twisted nematic (TN) mode, in-plane switching (IPS) mode, and vertically aligned (VA) mode. These modes differ from one another in a number of properties including the initial orientation of the liquid crystals, the structure of the thin film transistor, polarized film, operating method of liquid crystal, etc. For example, in a conventional TN mode, the liquid crystal is initially oriented so that is twisted by 90.degree. and an electric field is formed between the upper glass electrode and the lower glass electrode. In contrast, in the IPS mode, the liquid crystal is initially oriented horizontally and an electric field is generated parallel to the layer of liquid crystal. A VA mode LCD has a relatively high contrast ratio and a wide visual angle, as compared to a TN mode LCD. The liquid crystal molecules in a VA mode LCD normally appear black due to an initial orientation vertical to the glass electrodes.
A TN mode LCD has a relatively narrow visual angle, because the liquid crystal molecules are oriented in a particular direction by an electric field. On the other hand, an IPS mode LCD has a relatively wide visual angle, because the liquid crystal molecules rotate parallel to the glass electrodes to produce a small refraction difference in the liquid crystal molecules, and because phase retardation of light passing through the LCD cell is compensated. However, when a rubbing process is applied to an IPS mode LCD, a number of problems can result. First, unwanted backlight transmission through the LCD cell can occur due to non-uniformity of liquid crystal molecules, which is in turn caused by poor orientation of the orientation film. This is referred to as "leaking" of light. Although leaking can be prevented in a color filter process by use of a black matrix, this can cause other problems such as, for example, poor brightness or contrast ratio. Second, an IPS mode LCD has a poorer response time than a TN mode LCD, which is believed to be due to interaction between liquid crystal molecules and the orientation film. Thirdly, an IPS mode LCD has an image sticking which is not observed in a TN mode LCD, which is believed to be due to the structure of the cell and the interaction between liquid crystal molecules and the orientation film.
Generally, a conventional TFT LCD cell is composed of a glass substrate, a thin film transistor device, an indium tin oxide (ITO) transparent electrode and an orientation film. For a TFT LCD to operate, liquid crystal molecules should be initially oriented between the TFT LCD cells. The degree of orientation of the liquid crystal is the most important factor influencing the quality of the LCD. The degree of orientation of a liquid crystal is indicated by the "pretilt angle". The pretilt angle is a fixed angle by which liquid crystal molecules on the surface of an electrode are tilted with respect to the surface. The pretilt angle is dependent on a number of factors including composition of the liquid crystal, type of alignment layer, and rubbing strength. Generally, a TFT-LCD requires a pretilt angle of at least about 1.degree. to 3.degree., and a STN-LCD (super twisted nematic-liquid crystal display) requires a pretilt angle of at least about 5.degree..
In recent years, a method of preparing a TFT LCD without using a rubbing process has been developed, called a "non-rubbing process". In the non-rubbing process, a photopolymer is used as the orientation film material. Some examples of the non rubbing process are disclosed in Jpn. J. Appl. Phys., Vol 31, 1992, 2155; U.S. Pat. No. 5,464,669 to Dae S. Kang et al.; and Jpn. J. Appl. Phys. Vol. 34, 1995, L1000. The method disclosed in the Kang patent uses a polyvinyl 4-fluorocinnamate polymer to form an orientation film. A photopolymer contains photosensitive groups which, when exposed to linearly polarized ultraviolet light, respond so that the main chains of the photopolymer are oriented, thus causing orientation of the liquid crystal molecules.
Conventional cinnamate containing photopolymers such as polyvinylcinnamate have poor thermostability, i.e. they soften significantly at the glass transition temperature (Tg). This can destroy orientation of the polymer, thereby destroying orientation of the liquid crystal molecules. Also, because the sealing process for binding two glass electrodes in fabricating an LCD is carried out at an elevated temperature, the photopolymer used in the orientation film should be able to withstand temperatures of at least about 100.degree. C. Furthermore, the conventional cinnamate containing photopolymers do not provide sufficient resolution of moving images, because they do not have a sufficient pretilt angle.
With conventional cinnamate-containing photopolymers used in orientation films for LCDs, the alignment can be broken by a slight change in temperature due to the relatively low glass transition temperature (T.sub.g) of the main polymer chain and the side chains. Furthermore, the alignment of the liquid crystal molecules can be broken by a slight impact or an environmental change due to the relatively weak surface stability of the liquid crystals and the orientation films.
In order to overcome the shortcomings of the conventional cinnamate-containing photopolymers, the present inventors have developed new cinnamate-containing photopolymers which can provide improved thermostability, photoelectric characteristics, and pretilt angle of liquid crystal molecules for an orientation film for liquid crystal display. The cinnamate containing photopolymers can be applied to an LCD of a TN (twisted nematic) mode, an IPS (in-plane switching) mode or a VA (vertically aligned) mode.